Ensure XHR async is always true

This fixes #51

Allow beforeSend to be async & allow access to xhrFields

This release covers two issues:

1. beforeSend is now allowed to run async. If it returns a Promise, it will be waited on before sending.

2. beforeSend is now invoked after the xhrFields option has been processed such that the following assertion fails

See #52 for more details.

v2.1.1

Handle error thrown when parsing responseText as JSON #47 (thanks @ivospinheiro!)

v2.1.0

Add "use strict" #40

2.0.2

Removes a few items from .npmignore that are needed for the canjs/canjs tests.

2.0.1

Removes a few items from .npmignore that are needed for the canjs/canjs tests.

v1.3.0

Add "use strict" #40

v2.0.0

Features:

• parse rejected response #32
• implement beforeSend #34

Let XHR handle FormData object

#31

v1.1.4

Change the can-globals semver range to <2.0.0 #21